Excess biuret, a common impurity in urea fertilizers, is toxic to plants. Little is known about the mechanisms of biuret toxicity in plants. A previous study had shown that transgenic rice ( Oryza sativa) plants overexpressing bacterial biuret hydrolase improved biuret tolerance. Additionally, the biuret hydrolase-overexpressing plants showed a higher ¹⁵N ratio than wild-type plants when the roots were fed ¹⁵N-labeled biuret. Here, we determined biuret accumulation in rice seedlings by directly measuring the biuret. We found that the biuret hydrolase-overexpressing plants did not contain biuret, whereas wild-type plants accumulated biuret in shoots in the order of mmol L ^-1 tissue water. We also found that the concentration of allantoin, a nitrogen-rich intermediate compound in the purine degradation pathway, in rice shoots under biuret toxicity was higher than those in control conditions. Inhibition of allantoinase activity by biuret was not detected, and allantoin accumulation appeared to be associated with changes in the expression of putative allantoin transporter genes. Furthermore, another nitrogenous compound citrulline, which is a nonprotein amino acid, accumulated in rice suspension cells under biuret toxicity. The accumulation of these two nitrogen-rich compounds suggests that rice plants subjected to biuret toxicity may need to reduce the generation of surplus ammonium ions.